As disclosed in Korean Patent Publication No. 10-1063528 on 1 Sep. 2011, a conventional drum washing machine includes: a tub that is rotatably disposed in the inside of a case; and a motor that is disposed in the rear of the tub and whose rotating shaft passes through the rear center of the tub to then be connected to a drum, in which the motor includes a bearing housing that is fixed to the tub and for rotatably supporting the rotating shaft, a stator fixed to the bearing housing, and a rotor arranged at a predetermined gap on the outer circumferential surface of the stator and connected to the rotating shaft.
Such a conventional drum washing machine is configured so that when power is applied to the stator, the rotor rotates and the drum is connected to the rotor via the rotating shaft also rotates, to thus perform washing, rinsing, and dewatering strokes to wash laundry.
Generally, the number of slots provided in stator cores (that is, the number of teeth) in a motor for a washing machine is set in proportion to the number of poles (that is, the number of N-poles and S-poles) therein. When the number of slots increases, the washing machine motor exhibits high-torque and low-speed characteristics. Reversely, when the number of slots decreases, the washing machine motor exhibits low-torque and high-speed characteristics.
Meanwhile, the operation modes of the washing machine may be classified into a washing mode, a rinsing mode and a dehydration mode, that is, a spin mode.
The washing mode (hereinafter referred to as including the rinsing mode) requires a low speed (RPM) and high torque, and the dehydration mode (or the spin mode) requires a high speed and a low torque. That is, it is desirable to have the more number of slots at the washing mode in the washing machine motor, but it is desirable to have the less number of slots at the spin mode therein.
However, since the washing mode and the spin mode require opposite characteristics to each other, it is difficult to design the stator to have a structure, that is, the number of slots, to satisfy all of the requirements of the washing mode and the spin mode.
Therefore, a typical washing machine motor is designed to fit in the low speed and high torque in a washing mode, by using a concentrative winding method of winding coils around teeth provided in stator cores, but has caused the efficiency to be lowered when performing the spin mode. In addition, the operating time for the washing mode of the washing machine is very longer than the operating time for the spin mode thereof.
Therefore, the washing machine motor is designed to have the more number of the stator slots to be suitable for typically a washing mode, and to be optimized for the high-torque and low-speed characteristics for the washing mode, and to perform a control of weakening a magnetic field at the spin mode the low-torque and high-speed characteristics.
However, the above-mentioned magnetic field weakening control requires a very complex control circuit and control routine, and may cause a problem that cannot maximize the drive efficiency of the motor for washing machines.
Since driving efficiency of the motor for washing machines cannot be optimized in terms of the overall design in the case of designing a washing machine motor in consideration of only any one of the washing mode and the spin mode as described above, it is a common practice to design the washing machine motor to operate at an intermediate stage by considering a torque value of the washing machine motor required in the washing mode and a rotation speed of the washing machine motor required in the spin mode. As a result, there is a problem that the motor for washing machines cannot be designed to have a structure with the maximum efficiency at the washing mode or spin mode.
Considering it is difficult to increase the efficiency of the washing machine motor due to the above-described motor structure, Korean Patent Application Publication No. 10-2007-0066093 disclosed the optimal efficiency of the washing machine by using a motor drive method.
That is, the conventional motor drive method of the drum washing machine disclosed in the Korean Patent Application Publication No. 10-2007-0066093 is configured to include the steps of: determining whether operation information of the drum washing machine is a normal mode or a high speed mode when the drum washing machine starts to operate; and reading out square wave motor drive information at the normal mode, and applying the square wave motor drive information to the drum washing machine motor, and reading out sinusoidal wave motor drive information at the high speed mode, and applying the sinusoidal wave motor drive information to the drum washing machine motor.
Accordingly, the conventional drum washing machine was designed to improve the efficiency by applying the square wave motor drive method to the motor at the wash and rinsing modes requiring the low speed and high torque characteristics, and applying the sinusoidal wave motor drive method to the motor at the hydration mode requiring the high speed and low torque characteristics, but confronted a limit to the improvement in efficiency of the motor with no structural improvement of the motor.
In addition, alternatively, a direct and parallel drive method can be applied for controlling the motor for washing machines. The direct and parallel drive method is configured to employ a method of aligning stator coils as a direct drive method at the washing stroke, to thus increase the torque of the motor, and as a parallel drive method at the dehydration stroke, to thus increase the speed of the motor. However, in order to drive the stator coils in one of the direct and parallel drive methods according to the kind of the operation mode such as the wash and dehydration mode, a lot of switching elements are needed and a lot of peripheral circuits are added to increase the number of the circuits, to thereby increase the cost.
In addition, in the case of designing a stator to have a multi-slot structure to increase the motor efficiency at the time of a washing stroke, the coil winding volume increases and thus a back electromotive force (Back EMF) value is greatly increased. In this case, the motor can increase the speed up to the back EMF value becomes the direct-current (DC) power voltage. Thus, the motor of the multi-slot structure may have the difficulty in securing the dehydrating voltage margin at the time of the dehydration mode requiring the high speed.
Meanwhile, in general, in the case of using a motor with a single stator as a drive apparatus for a drum washing machine, if the motor is designed as a structure suitable for only any one of the washing stroke and the dehydration stroke, the problem of lowering the efficiency of the motor at the other stroke cannot be fundamentally solved, since the rotational speeds and torques of the motor required in the washing stroke differ from those of the dehydration stroke. However, the inventors have found that the motor can be designed with the possible best efficiency at both the washing stroke and the dehydration stroke in the case of using a motor of the double-stator and double-rotor structure.